Pour 3 quarts of water into a large, heavy-duty roasting pan set over two burners or into a large kettle, such as a lobster or crab pot. Add onions, garlic, seafood seasoning (yes, 1/2 cup is the correct amount) and salt to taste. Squeeze lemon juice into the stock, then toss lemon halves into the pan or pot.

Bring to a boil over medium-high heat. Add potatoes and kielbasa; return to a boil. Cover pan with foil or partially cover pot with lid. Reduce heat and simmer, cooking until potatoes are just tender, 20 minutes.

Add corn; increase heat to medium-high and cook, covered, until tender, about 5 minutes longer. Stir shrimp into hot broth; let stand, covered, until they are just cooked, about 2 minutes longer.

Serve seafood, sausage and veggies on one or two large platters, with lemon wedges and small bowls of melted butter for dipping.